Bio-Electrochemical Studies for Harvesting Carbon Dioxide to Organic Compounds

Abstract

Bio-Electrochemical Studies for Harvesting Carbon Dioxide to Organic Compounds Powering microbes with electricity in bio-electrochemical synthesis (BES) to produce plethora of organics like volatile fatty acids and alcohols is an attractive bio sustainable strategy to minimize our dependence on fossil fuels. Bio-electrochemical synthesis, a beneficial key technique in which electro autotrophic bacteria utilize electric current as a sole electron source from cathode to reduce CO2 to extracellular multicarbon exquisite products through metabolic conversion. The anaerobic and autotrophic acetogens like Sporomusa ovata, Clostridium ljungdahlii and Cupriavidus necator have been focused in current study to convert waste greenhouse gas like CO2 into volatile fatty acids, alcohols and Polyhydroxyalkanoates (PHA). These electroactive catalysts were able to capture electron directly from cathode without any mediator due to the presence of C-type cytochrome and type IV pili in Sporomusa ovata, RnF complexes for electron and proton translocation in Clostridium ljungdahlii and Fln type adhesion on the surface of Cupriavidus necator. The bio-electrochemical reactor was simplified to avoid time expenditure by eliminating potentiostat and by improving start-up process of autotrophic biocathode. The two stage strategy was integrated in this system based on heterotrophic pre-enrichment of electro autotrophic biocatalysts on glucose or fructose, afterwards acclimation of pre-enriched culture to BES reactor where CO2 was sole carbon source to switch bacteria from heterotrophic to autotrophic metabolism. The biocathode was poised at -0.4V, (versus Ag/AgCl electrode) high enough to avoid hydrogen production by DC power source other than potentiostat to retain the high coulombic recovery in electro fuel production. The development of pure microbial biofilm at cathode rather than mixed culture further simplified the BES reactor from complex metabolic activities. The BES technique was practiced first time to synthesize high quality Polyhydroxyalkanoates (while fermentation process was followed before) merely from cheap and low cost substrates like molasses and waste greenhouse gas CO2. Implementation of simplified reactor and specific strategy for both batch and continuous system under ambient conditions of pH, temperature and pressure enhanced the electroactivity of cells to transform the electrons to spectrum of xi extracellular products in less time duration. The net outcome was the renewable energy which was stored in covalent bonds synthesized from waste greenhouse gas. The specific products included acetate, butyrate, ethanol, hexanoic acid, hexanol, heptanoic acid and heptanol by Clostridium ljungdahlii and Sporomusa ovata. The formation of heptanoic acid and heptanol in this specific technique never reported earlier. This feature has potential to make these electrotrophs beneficial for biotech industry. The intracellular Polyhydroxyalkanoates accumulated as granules inside the cells are environment friendly thermoplastics. The novel extraction techniques for the recovery of granules of carbon and energy reserves from cell dry mass has made PHA more attractive. Gas chromatography and mass spectrometry (GC-MS) analysis employed for identification and quantification of electro fuels and further electroactivity was characterized by cyclic voltammetry. The ANOVA test performed for statistical analysis between the batch and continuous system for all three strains. ANOVA test for Sporomusa ovata proved the significant difference between batch and continuous system rather than other two strains. The overall coulombic recovery was more than 90% due to the redox electroactivity of these autotrophs. The concentrations for ethanoic acid, ethanol, ethyl butyrate, hexanoic acid, heptanoic acid, hexanol and heptanol were 2.99, 3.19, 2.2, 2.18, 2.01, 2.11 and 0.85mM at 120 hours of medium cultivation. The Coulombic recovery was more than 80% proved the BES a promising and remarkable technology than other chemical and photosynthetic based chemical production.